1. Field of the Invention
This invention relates generally to methods and apparatus for cleaning and coating pipeline walls, and in one particular aspect, but not by way of limitation, to pipeline pigs adapted to be propelled through a pipeline, such as an oil or gas pipeline, by a pressure gradient.
2. Brief Description of the Prior Art
Various devices and methods have been used to internally apply cleaning and coating materials to pipe walls. Many of these devices are unsuccessful in coating all portions of the pipeline wall. Those methods and devices which do enjoy some level of success at coating all portions of the pipe are expensive to use because of the excess amount of coating material that is wasted in the effort to assure that all protions of the pipe have been coated. Some methods waste 50% of the coating material as compared to the amount of material that would be necessary to do the job satisfactorily.
A customary pipeline coating technique now in use is to provide a pair of spaced pipeline pigs which define between them a space axially extending in the pipeline and blocked at each end by the spaced pigs. Pressure is applied to the trailing pig to drive the paired pigs through the pipeline with a liquid used for coating the pipeline placed in the space in between. At times, the leading pig will tend to run away from, or move faster through the pipeline than, the trailing pig. When this happens, the liquid between the two pigs does not completely fill the intervening space, and the result is that there are holidays left on top of the pipeline where the liquid does not contact it and effectively coat the pipeline.
In attempting to regulate the speed of a pig to prevent it from running too fast, pressure is built in front of the pig and held as a back pressure. A greater pressure is applied to the back (or rear end) of the pig so that the pig moves forward in response to the forward force resulting from the pressure gradient between the rear and the front of the pig. The use of this technique to drive pipeline pigs has also resulted in the unsatisfactory coating of pipeline walls because when this technique is used to propel a pipeline pig through the pipeline, there is a tendency for the pig to tilt forward. When the pig is so tilted, it loses contact with part of the pipeline wall, thus preventing the uniform coating of that wall.
A discussion of all the prior art in this area would be prolix and prohibitively long; therefore, only the most pertinent patents will be discussed.
U.S. Pat. No. 3,112,227 to Curtis shows a rotary device used for applying paint or other protective compounds to the interior of conduit. The device is mechanically driven through the conduit by the use of an elongated tubing and the rotary force is also applied by means of a torque or rotating force applied to this tubing attached to one end of the rotary device used for applying the coating. The coating material is directed through the hollow interior of the driving tube to a central body, and from this body the coating material is distributed outwardly through radial pores formed in the center portion of the body.
U.S. Pat. No. 4,069,535 to Cato discloses a pipeline pig composed of an elongated cylindrical body with a front end and a rear end, and including a plurality of spaced, annular, flexible rings traversing the cylindrical body. The Cato patent also discloses a pig with a hollow interior which is open at the rear end to allow fluid or gas under pressure to enter the pig. Therefore when the pig is placed in a pipeline and pressure is applied behind the pig, it is driven down the pipeline.
U.S. Pat. No. 4,081,875 to Nishino discloses a scale removal device for scraping off scale deposits on the inner surface of a pipe. The device includes a rotary shaft which is positioned within the tube for advancement and rotation under the action of fluid under pressure pumped into such pipe. The rotary shaft includes a spiral blade extending along a substantial length of the shaft. Fluid is pumped against the device, causing forward thrust and making the shaft rotate. The rotation of the shaft thereby causes the spiral blade to scrape against the pipe, producing the cleaning action desired. The spiral blade and the inner wall of the pipe are so positioned that fluid or other liquid may pass between the blade and the pipe.
U.S. Pat. No. No. 4,175,750 to Nichols also relates to a tube cleaner. The Nichols tube cleaner includes a spirally configured member which is caused to rotate to clean the tube's interior walls. This device also allows fluid to pass through the pipeline past the device and the interior walls.
U.S. Pat. No. 4,122,575 discloses a tube cleaning device which has a compressible elastic body made of material such as polyurethane. This material has at least one spiral groove formed in its periphery running from the front to the rear side or end of the body. When a force resulting from a pressure gradient is applied to the material, a thrusting force is applied to the inclined surface of the spiral groove, causing the device to rotate.
As can be seen, the prior art has developed devices which usually require the use of either a force resulting from a pressure gradient or an external rotary force so that they may be propelled down the pipeline. As previously discussed, the use of a force resulting from a pressure gradient to propel a pig results in an unsatisfactory coating of the pipeline walls. The use of external rotary force, as can be appreciated, severely limits the types of pipeline wall coating tasks that can be undertaken. Only the Cato patent discloses a pipeline pig which utilizes pressure acting interiorly of a pig to facilitate movement through the pipeline, thereby avoiding some of the problems created by the designs of the prior art. The Cato patent, however, in contrast to the present invention, locates a plurality of annular rings along the elongated body of the pig. This design requires the user to utilize an excessive amount of material if he wishes to insure that all portions of the pipeline wall are coated. This is so because the annular rings only act to smooth the material onto the pipeline walls, and therefore require a column of material in front of them which contacts all portions of the wall if the entire wall is to be coated. In contrast, the present invention utilizes helically oriented flights which will pick up material from the bottom of the pipeline and will carry the material to all areas of the pipe. Not only does this design save the manufacturer material and therefore money, but it also facilitates the uniform coating of pipelines in areas where the prior art was previously unsuccessful. Because of the construction of a pipeline, and the topography in which it is often laid, there are times when a full column of coating material cannot be supported by the pipeline coating devices and therefore portions of the pipeline wall are not coated. The present invention, because of its design, can uniformly coat a pipeline wall, even if a minimal amount of coating material is present.